The present invention relates to the fabrication of semiconductor devices, and particularly to the temporary adhering together of device parts in self-aligned relationships within stacks of device parts prior to and during formation of soldered joints between the parts.
In a known semiconductor device fabrication process, a number of device parts, including intervening discs of solder, are loaded one on top of the other in a jig cavity and the jig is then heated to melt the solder discs to form a stack of parts that are bonded together. The loaded parts can comprise, for example, a first terminal lead, a first solder disc resting on a surface of the first lead, a semiconductor chip (or die) resting on the first disc, a second solder disc on the chip and, finally a second terminal lead contacting the second solder disc.
During the fabrication process, the jig is moved along an assembly past successive work stations where the individual parts are sequentially loaded into the jig and the loaded jig then passed through a furnace for heating and reflowing the solder. A problem that arises is that the movements of the jig and vibrations of the work stations can cause lateral displacements of the parts within the jig cavity, resulting in defective device parts bonding. Another problem is that the jig is typically made of material that is resistant to high temperatures such as aluminum, graphite, or copper-graphite. The large thermal mass of the jig may adversely affect the reflow process. For example, the thermal mass sometimes causes inconsistent, highly variable results.
An alternative approach is to provide the solder in the form of an adhesive paste, e.g., a mixture of solder particles in an adhesive matrix having a paste-like consistency. One advantage of this approach is that a jig is generally not required because the adhesive paste is relied upon to maintain parts alignment. Problems with such pastes, depending on the paste used, is that additional cleaning processes may be required to remove the matrix after completion of the soldering process. Moreover, the adhesive strength of such pastes is often not sufficiently great to maintain parts alignment during the solder reflow process. In general, while the use of solder in the form of an adhesive paste is widely used and often satisfactory, there remains a need for an alternative and improved self-adhesive parts stacking process.